It's only natural to assume that after you've gone through the complex process to obtain a visa to travel to the U.S., that you would be guaranteed entry when you finally arrive. Unfortunately, that is not the case. You are not guaranteed entry into the United States simply because you hold a visa. The Department of Homeland Security, U.S. Customs and Border Protection officials make the final determination whether or not to admit you into the country.
Arrival
When you arrive, you will be enrolled in the US-VISIT entry-exit program. Some people will also need to register with the Special Registration program. If the Border Protection official allows you to enter the U.S., they will add an Arrival-Departure Record (Form I-94) in your passport. The dates of your approved visit will appear on this form. It is extremely important that you keep this form in your passport, as it is the only record you have of entering the country legally.
Make note of the end-date on the I-94. You must leave the U.S. by that date to avoid being out-of-status, unless you have the option to renew your visa or adjust your status before the expiry. If you stay beyond your end-date--by even one day--you will be deemed out-of-status. Your visa will be voided and you will be in violation of U.S. immigration laws.
Preparation
Have all your documentation in order and be ready to present it to the Border Protection official when you arrive at a port of entry. The Border Protection officials have the right to inspect your documents, search your belongings and ask you questions about your travel and intention to enter the U.S.
Don't be nervous if you're stopped for inspection. The Border officials act as the front line for America's borders. In addition to being responsible for homeland safety and security, they also enforce the laws of other federal agencies. While you might feel intimidated by the questions and the scrutiny, these men and women are professionals who only want to protect our borders. Be pleasant and patient with the process, and in no time you'll be on your way.
Source: The U.S. Department of State
